The invention refers to the production of valved synthetic resin bags, provided with profiles or shapings, cuts and laminar parts, internally or externally applied, or with foldings, to constitute retaining valves of the bag contents and to permit filling the bag with loose material. The production of this type of bag is known--according to a possible embodiment--by means of the application of welded thicknesses or linings forming a pocket; with an incision on the wall of the bag, the retaining and filling valve is thus completed.